DEVONthink 3 is a knowledge-base application that allows you to store, sort, tag, and retrieve any kind of data that you can think of. I personally like storing all of my text-based notes in DEVONthink as Markdown documents. You can specify a CSS file to be used to style the markdown. It's super easy to make your own too. Just grab this theme (CSS file) and hack it until you have something that you like. Good luck!
Download the theme here.
Created by Patrick H. Mullins. Source is released under the MIT license. Please report any bug or issues as an official GitHub issue here. You can find me on Twitter and on Telegram as @pmullins.